Referring to FIG. 1, a conventional socket tool for mounting removably a screwdriver head 10 includes a rod body 11 having an insert recess 111 and a front end opening 112, a sliding sleeve 12 sleeved movably on the rod body 11, a sliding member 13 disposed movably in the insert recess 111, a spring 14 abutting against the sliding member 13, two positioning balls 15 contactable with the screwdriver head 10, and a retaining ball 16 contactable with the sliding member 13.
When removal of the screwdriver head 10 is desired, the sliding sleeve 12 is moved forwardly on the rod body 11 to remove the positioning balls 15 and the retaining ball 16 from the screwdriver head 10 and the sliding member 13, respectively, thereby allowing the screwdriver head 10 to be removed from the rod body 11, as shown in FIG. 2. However, after a long time period of use, wear is experienced by the retaining ball 16, thereby resulting in difficulties in positioning effectively the sliding member 13 in the rod body 11. Moreover, the retaining ball 16 is located at one side of the rod body 11 so that a retaining force is applied to the sliding member 13 by the retaining ball 16 from only one lateral side of the rod body 11. As a result, the sliding member 13 cannot move smoothly, thereby affecting adversely quick removal of the screwdriver head 10.